Workpieces, such as semiconductor wafers or solar cells, typically require multiple process steps to fabricate a finished product. For example, besides forming a p-n junction in a solar cell, material layers may be deposited on the solar cell to enhance performance or improve efficiency. Texturing the surface, cleaning, or adding metal layers to the surface of the solar cell also may be performed.
“Black silicon” may be used in solar cells. This is a specific type of silicon that absorbs a large percentage of the solar spectrum and can enable more efficient solar cells. It is referred to as “black” because it appears black to a viewer due to the wavelengths absorbed and its low reflection. The high light absorption of black silicon is due to its particular surface texture. FIGS. 1A-1B are images of one type of black silicon or textured silicon. FIG. 1A is a scanning electron microscope (SEM) image of black silicon. The textured pattern seen in FIG. 1A is created by femto laser-based fluorinated chemistry etching of the silicon surface. FIG. 1B is the surface of a textured silicon workpiece. Texturing the surface with, for example, pyramid shapes increases light collection. Known wet chemistry processes, however, are limited by the crystal plane of the workpiece being etched and the etch rate of a particular crystal plane. Besides this limitation, wet etching, such as the use of KOH, or plasma etching creates random pyramid patterns.
FIG. 2 is a cross-sectional view of a selective emitter solar cell. The selective emitter solar cell 200 has, in this particular embodiment, an n-type base 201 on a base 202. Near the base 202 are n++ regions 203. Opposite the base 202 is a p+ layer 204 and anti-reflective coating (ARC) 205. The ARC 205 may be SiNx in one instance. Metal contacts 206 are disposed in the selective emitter solar cell 200 near the p++ regions 207. The n++ regions 203, p++ regions 207, and p+ layer 204 may be formed by doping. The surface 208 of the selective emitter solar cell 200 may be textured similar to those illustrated in FIGS. 1A-1B. An oxide layer between the ARC 205 and p+ layer 204 may be used in one embodiment.
FIG. 3 is a cross-sectional view of an interdigitated back contact (IBC) solar cell. The IBC solar cell 300 in this particular embodiment includes an n-type base 301, an n+ layer 302, and an ARC 205. An oxide layer between the ARC 205 and n+ layer 302 may be used in one embodiment. Opposite the n+ layer 302 are alternating p+ emitters 303 and n+ back surface fields 304. A passivating layer 305 defines contact holes 306. Alternating n-type contacts 307 and p-type contacts 308 contact the p+ emitters 303 and n+ back surface fields 304. In the embodiments of FIGS. 2-3, the n-type and p-type doping may be reversed or modified.
The patterning of the layers or doping of particular regions in the solar cell, such as the selective emitter solar cell 200 of FIG. 2 or the IBC solar cell 300 of FIG. 3, and formation of metal layers or contacts may be an expensive process. The complexity or number of process steps may increase the cost of ownership and fabrication time. If at least some processes used for fabricating a solar cell or other workpiece were performed in the same system, throughput may be increased and cost per workpiece may be decreased. Eliminating or simplifying steps likewise increases throughput and decreases cost per workpiece. Accordingly, there is a need in the art for an improved method of fabricating a workpiece and, more particularly, of fabricating a workpiece by processing with a plasma focused by an insulating modifier.